It is the difference between the pythagorean or 3-limit "minor 7th" of approximately 9.96 Semitones [= 16/9 = 3-2 = ~ 996.089 cents] and the "harmonic 7th" of approximately 9.69 Semitones [= 7/4 = 71 = ~ 968.825 cents].

There are many other small septimal intervals which may be called commas, such as:

50/49 [= 527-2 = ~ 34.976 cents]

49/48 [= 3-172 = ~ 35.697 cents]

36/35 [= 325-17-1 = ~ 48.770 cents]

225/224 [= 32527-1 = ~ 7.712 cents]

However, only the first two listed fall within the range of what is properly a "comma" -- it is more appropriate to call 36/35 a "septimal diesis", and to call 225/224 a "septimal kleisma", based on their size-relationships to other intervals with those names.

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